Northern red-legged frog (Rana aurora)

Physical description

This is a medium-sized frog with a slender body, smooth skin, distinct dorsolateral folds, and an eye mask. Females can reach 4.2 inches snout to vent length and males 3.2 inches snout to vent length but in Washington females rarely exceed 3.5 inches and males rarely exceed 3 inches. The dorsal (topside) color is tan, brown, or olive-brown with varying amounts of black spotting and speckling. The undersides of the legs and margins of the abdomen are brick to orange-red; a gray wash may also be present. The legs are long. Mature males have gray swollen pads, called nuptial pads, on the thumbs. Juveniles resemble adults but may not have the red pigment on undersurfaces

The presence of dorsolateral folds distinguishes all native True Frogs (Ranid species) from other anurans in Washington State. All native True Frogs in western Washington are brownish colored frogs that are similar in general appearance. Cascades frogs are easily distinguished by the honey-yellow coloration on the abdomen and undersides of the legs.

The following northern red-legged frog traits distinguish them from Oregon and Columbia spotted frogs: 1) the groin is mottled with black and greenish yellow or cream-colored blotches, 2) distinct dorsolateral folds are present along the entire dorsal margins, 3) the eyes are oriented outward (laterally) so that only a portion of the pupil is visible when the frog is viewed from above, and 4) webbing is reduced between the toes so that the webbing on the longest toe does not extend past the first joint and the webbing is concave when the toes are pulled apart.

The exotic American bullfrog and exotic American green frog have a distinct fold (supratympanic fold) from the posterior edge of the eye, around the top of the external ear (tympanum) and ending at the shoulder; and typically have green coloration on the head.

Larvae

The tadpoles of the northern red-legged frog, Oregon spotted frogs, Columbia spotted frogs, and Cascades frogs are similar enough that distinguishing them is challenging.

The red-legged frog tadpole has an oval body with dorsal eyes. The tube for waste elimination (vent) is just right of the midline at the tail base, and the tube for moving respiratory water out of the body (spiracle) is on the upper middle left side. The dorsal fin terminates on the body at, or anterior to, the spiracle. The tail tip is pointed, and the dorsal fin is at least as high as the height of the tail musculature.

At hatching, tadpoles are dark brown but appear black in most light. They have long tails and short gills. As the larvae grow, the gills become concealed, the body color and belly lighten, and metallic flecks appear. In larger tadpoles, the ventral abdomen is often pinkish or yellowish and brassy flecks are visible on the sides. Tadpoles grow to 2 to 3 inches total length before metamorphosis.

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Photo by W.P. Leonard, Copyright

Pacific treefrog tadpoles differ in having eyes along the outline of the head when viewed from above. https://wdfw.wa.gov/species-habitats/species/pseudacris-regilla

Western toad tadpoles are black dorsally and, ventrally, have minimal if any metallic flecking, have flattened bodies, have a tail fin that does not extend onto the back, and have narrowly placed dorsal eyes on top of the head.

Large American bullfrog and American green frog tadpoles have distinct black spots on a khaki-colored body and the abdomen is an opaque yellow (intestines not visible) with no metallic pigment (bullfrog) or with a coppery sheen (green frog).

Eggs 

Egg masses are globular with jelly that is soft and pliable to the touch. Roughly the size of a grapefruit, they are attached to vegetation 6 inches or more below the water surface. Newly laid eggs appear black on top and white on the bottom. As the embryos develop, the jelly becomes looser and the egg mass tends to break free of its attachment, often flattening out and floating to the surface. Newly hatched tadpoles disperse from the egg mass soon after hatching.

The Cascades frog, Oregon spotted frog and Columbia spotted frog typically lay their eggs in communal clusters that consist of many (up to 60 or more) egg masses laid next to or on top of each other in shallow (less than 6 inches) water and the eggs are not attached to sticks or vegetation.

The northwestern salamander has a fist-sized globular egg mass that is solid to the touch, always attached to vegetation and the animal pole of the egg and the embryos are brown not black or dark brown.

Voice 

The advertisement call is a weak, stuttering set of guttural notes (4 to 7) given on one pitch “uh-uh-uh-uh.” This call, produced primarily underwater, requires one to be close to the calling male to hear it. A chorus sounds like soft “clucking.” Individual males also produce infrequent low volume calls, “chuckles,” from terrestrial sites during the active season. The release call is “chuckling” accompanied by pulsing of the body. When grabbed by certain predators, such as common garter snakes, northern red-legged frogs release a loud and startling “scream.” Listen to the calls of northern red-legged frog in this short audio clip.

For more details about red-legged frog, see the Washington Herp Atlas.

Ecology and life history

Northern red-legged frogs are found in lowland (mostly below 3000 feet in Washington) moist forested habitats with access to suitable breeding sites. This species can persist in areas of low-density development and landscapes managed for timber. Transformed northern red-legged frogs are the most terrestrial of our native True Frogs. It is not unusual to find them in moist forested habitats far from water bodies. However, most individuals are found in the vicinity of standing or flowing water, even during the non-breeding terrestrial phase of the seasonal life cycle. Juveniles have been found in road puddles in disturbed open habitats that would not typically be considered suitable habitat for the species. Puddles and other temporary sources of standing water may be important features for dispersing individuals.

The few data obtained on overwintering for northern red-legged frogs in the Puget Lowlands of Washington have revealed that the species may overwinter in terrestrial habitat at least for part of the season. Overwintering data from the lower Columbia in Oregon indicate that frogs will shuttle between aquatic and terrestrial habitat depending on wintertime temperatures, moving into water when conditions are colder. Regardless of precise overwintering location, this species can remain active throughout the winter at low-elevation sites except when temperatures are near or below freezing.

During periods of inactivity, northern red-legged frogs have been found sheltering under or within sword fern fronds, the duff that accumulates below sword ferns, under woody debris and within stream banks.

Northern red-legged frogs can be active both day and night depending on temperature; during wet intervals, most activity takes place at night. They blend well into their surroundings and are difficult to detect unless they move. They are alert, fast frogs that will remain motionless when approached and then suddenly flee by leaping away with relatively long but evasive jumps.

Breeding habitats include a variety of still-water bodies that generally persist until at least July at low elevations, later at high elevations. Vegetation suitable for egg mass attachment must typically also be present. However, northern red-legged frogs can sometimes lay eggs in mud puddles with no attachment brace, but this pattern is infrequent. Larvae are grazers and can strongly influence the distribution of algae (periphyton) in some still-water habitats. In water bodies with introduced warm-water fish, egg masses are uncommon or absent suggesting that northern red-legged frogs avoid these water bodies for egg laying.

Breeding in Thurston County, at 50 to 100 feet elevation, starts in late January or early February. Most eggs are laid within 2 to 3 weeks at each site, but a small percentage of populations, especially large ones, will continue to lay over a longer period of time. In southwestern British Columbia, eggs take approximately 3 to 5 weeks to hatch depending on when they are laid, water depth and water temperature. Oviposition appears to occur when surface water temperatures have reached 43° Fahrenheit. Most egg masses have hatched by late March in lowland areas. Tadpoles at low elevations start to transform in late June of their first year.

Experimental studies have revealed that exotic warm-water fishes can interact with bullfrogs and other aquatic fauna in a manner that produces negative effects on northern red-legged frogs. Overall, however, habitat structure and the presence of exotic fishes appear to be more important in determining the overall distribution and abundance of northern red-legged frogs than simply the presence of American bullfrogs.

Geographic range

In Washington, northern red-legged frogs occur in the Pacific Coast, Puget Trough, North Cascades, West Cascades and East Cascades ecoregions. East of the Cascade crest, the only records from Kittitas and Yakima Counties are old museum specimen records.

This map from the Washington Herp Atlas illustrates the distribution of northern red-legged frog in Washington based on records in the WDFW database as of 2016. If you see this species in areas that are not indicated on the map or have more recent observations (less than 10 years), please share your observation using the WDFW wildlife reporting form.  

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For a map of range-wide distribution and conservation status of this species, check out NatureServe Explorer and the International Union for Conservation of Nature Redlist.